The present invention relates to electronic devices and methods for making them employing alloys of compounds having constituent elements in Group III and Group V of the periodic table.
The development of new and useful electronic devices in large part depends on the discovery of new materials and convenient and inexpensive methods of fabrication. One area in which research in electronic devices is presently proceeding apace is in optical communications technology. Transmission of information at high rates over beams of light conducted through low loss optical fibers offers practical and economic advantages compared to wire and radio transmission techniques.
An optical fiber link typically involves a light transmitter at one end of a light path and a light receiver at the other end. The light transmitter typically incorporates a light-emitting diode or a semiconductor laser for producing light in rapid response to an electrical signal bearing the information to be transmitted. The light receiver incorporates one of a variety of types of photodetector such as a Schottky diode or p-n junction semiconductor photodiode so that the incoming light is converted to an electrical signal again.
In such devices, materials having constituent elements in Group III and Group V of the periodic table, among which gallium arsenide is perhaps best known, have proven especially useful in the forming of crystalline layers. Depending on the wavelength of light to be used in the infrared or visible regions of the spectrum and the function of the device, the energy gap between the valence and conduction bands of the III-V material is designed to permit emission or absorption accordingly.
Gallium arsenide antimonide (GaAs.sub.1-x Sb.sub.x), which is a solid solution of the binary compounds gallium arsenide (GaAs) and gallium antimonide (GaSb), has an energy bandgap between about 0.9 micrometers and 1.7 micrometers of wavelength, depending on the relative proportions of the binaries. Since optical fibers exhibit low losses in this wavelength region and especially around 1.1 micrometers in the near-infrared, GaAsSb is exciting considerable interest for use in devices at the transmitting and receiving ends of optical fibers.
Unfortunately, however, it has proven difficult to grow GaAsSb on readily available substrates such as gallium arsenide so that the economic potential of GaAsSb can be realized in practical devices. The reason is that GaAsSb has a lattice constant different from substrates such as GaAs, introducing mechanical stresses and less than satisfactory crystal growth resulting in crystalline defects which shorten device operating lifetimes.
Heretofore, a thick layer of GaAsSb has been deposited on GaAs to reduce the effect of strain due to lattice mismatch between the substrate and subsequent grown layers, but the lattice mismatch with consequent disadvantageous effects remains. Another method has been to deposit a multiplicity of GaAsSb layers having differing compositions intermediate between the substrate and the active GaAsSb layer. Performance of devices is improved, but at the cost of complicating the crystal layer growth method and reducing the economic attractiveness of GaAsSb devices for optical communications applications.